May, 1922
THE JOURNAL OF INDUXTRIAL A N D ENGINEERING CHEiMIXTRY
2 hrs., and weigh, as the glyoxime salt, which contains 20.31 pdr cent nickel.1s . 16
In routine analyses the glyoxime precipitate may be filtered on paper,
the wet paper wrapped in another moist “ashless” paper and then slowly and carefully ignited t o NiO. It may also be dissolved in nitric acid and titrated with cyanide solution by the method of Frevert [A. A. Blair, “Chemical
429
ACKNOWLEDGMENT The authors desire to express their indebtedness to Dr: W. F. Huebrand for his interest and aid in the work. Analysis of Iron,” 8th edition, J. B. Lippincott and Co., 182. See also G. I,. Kelley and J. B. Conant, THISJOURNAL, 8 (1916),8041
The Analysis of Solutions of Ammonium By C. S. Robinson3 and Selma L. Bandemer MICHIGAN AGRICULTURAL COLLEGE EXPRRIMENT STATION,EASTLANSING, MICHIGAN
A n inoestigation of methods of determining the ratio of ammonia to citric acid in ammonium citrate solutions points to two methods which can be relied upon to gioe accurate results. When properly carried out they gioe values which agree with each other and accurately represent the composition of the solutions analyzed. I n the formaldehyde titration method, the only precautions to be obseroed are that the formaldehyde shall be neutral and present in suficient quantity to combine with all the ammonia. I n the other method the alkaline residue left after the distillation of the ammonia is made acid to methyl red with a measured amount of standard acid and titrated with standard alkali with phenolphthalein indkator.
N the course of the study of the colorimetric method
I
for the preparation of neutral solutions of ammonium citrate, undertaken while the senior author was associate referee on this subject for the Association of Official Agricultural Chemists, his attention was called to the diversity of procedures employed in the analysis of such solutions. As the determination of the ratio of ammonia to citric acid by analysis is one of the best ways of checking the reactions of ammonium citrate solutions the accuracy of the methods used is of considerable importance. While referee on phosphoric acid in 1908, McCandless4 used the determination of the ratio of ammonia to citric acid as the criterion of the reaction of solutions submitted to him. His procedure consisted of adding a measured amount of 0.25 N alkali to the citrate solution and distilling off the ammonia which was collected in a measured amount of standard acid and determined by titrating the excess acid. To the residue in the distillation flask he added “an excess” of 0.5 N acid and phenolphthalein and titrated back with 0.1 N alkali to the end-point. This procedure is apparently used in many laboratories either exactly as described by McCandless, modified as to the strength of solutions used, or by eliminating the last step and titrating the excess of the originally added standard alkali directly to a phenolphthalein end-point. A second type of method is the so-called formaldehyde titration method suggested by Patten and Marti.6 In this method the ammonia is removed from the sphere of action by causing it to combine with formaldehyde, as was done by Schiff.6 The citric acid is then titrated directly to a phenolphthalein end-point. Several collaborators claimed that they were unable to get these two types of methods to agree, and hence concluded 1
Received January 12, 1922.
No. 22 from the Chemical Laboratory of the Michigan Agricultural College Experiment Station. Published by permission of the Director of the Experiment Station. a Research Chemist, Michigan Agricultural College Experiment Station. 4 Bur. Chem., Bull. 122, 147; THIS JOURNAL, 6 (1914),921. 6 THISJOURNAL, 5 (1913). 568. a Published as Journal Article
9
Ann., 319 (1901),76.
that the second one was unreliable. Upon trying to check up the composition of some solutions by both types of methods the authors actually found that the results did not agree. However, as the formaldehyde method had been in use in this laboratory since it was first suggested and since it is in constant use in scores of other laboratories in biochemical work, the authors were skeptical as to its shortcomings. Hence the investigation of both methods was undertaken.
COMPARISON OF RESULTS The first points studied were the agreement of these methods with each other and with themselves when used on the same solution. Needless to say, every possible precaution against error was taken. The reagents were all carefully standardized, using as a standard of reference 0.1 N HC1 prepared by the method of Hulett and Bonner7 and checked gravimetrically. The flasks and burets were all carefully calibrated. (This precaution should not be neglected. Burets have been purchased recently from reputable supply houses which upon calibration have been found to be as much as 0.4 CC. off, and variations of 0.1 cc. are common.) The following procedures were studied: 1-The original technic of McCandless, with 0.26 N alkali for the initial stage, 0.5 N acid for the second, and 0.1 N alkali for the back titration. 2-The same technic as (l),but with 0 1 N reagents throughout. 3-The same procedure as ( 2 ) , except that the addition of acid was continued until the solution was acid to methyl red. 4-Direct titration to a phenolphthalein end-point with 0 5 N HCl, after the addition of 35.00 cc. 0.5 N NaOH. 5-The formaldehyde method, with 0.1 N reagents. The results are shown in Table I, page 430. At the bottom of each set of determinations is shown the maximum variation among the individual estimations. It will appear at a glance that two of the methods, viz., 2 and 4, give variations much larger than those in the other two methods. I n calculating these variations all estimations have been considered. The proponents of these procedures will undoubtedly point out that this is unfair, since in some case8 most of the figures agree fairly well, while only one or two differ materially from the rest. Thus, in the results by Method 2 on Solution I1 the second determination was a whole cubic centimeter lower than the next one to it. Were this result to be discarded, as, of course, it would be in practice, as being “off,“ the maximum variation would be reduced from 1.70 to 0.70. The same general statement applies to Determinations 4 and 5 by Method 4 on Solution I1 and 5 and 6 by the same method on Solution 111. These results are, 1
J . Am. Chcm. SOC.,31 (1909), 390.
T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
430
TABLEI-COMPARATIVE’
ELECTROMETRIC TITRATIONS
RESULTSWITH DIFFERENT METHODS
SOLUTION DETERMINATION 7 No. No. 1 ‘ I 1 2 3 4 ... Maximum variation, . . . . . . . . . . . . . . . . . AVERAGE., . . . . . . . . . . . . . . . . . .
... ...
...
PROCEDURE No. 2 3 4 26.01 26.27 26.19 26.17 26.32 26.22 26.46
...
0.45 26.25
0.10 26.19
30.23 29.23 30.93 30.73 30.53 30.38
... ... .... .. ...
5
i::::
::
...
26.14
27.27 27.48 27.48 27.48
... ...
30.70 30.05 30.05 31.05 29.55 30.05
27.20 27.24 27.30 27.30 27.24 27.30
1.70 30.34
0.21 27.38
1.50 30.24
0.10 27.25
27.40 27.30 27.38 27.53 27.55 27.51
31.02 31.85 32.35 31.72 32.25 29.64
27.08 27.58 27.08 26.93 27.29 27.18
28.46 28.26 28.71 28.67 29.36 27.07
27.03 27.08 27.03 27.03 27.08 27.03
Maximum variation., , , . , . , , . . , 0.25 AVERAGE., . . . . . . . . . . . . . . . 27.44
2.71 31.47
0.65 27.19
2.29 28.42
0.05 27.05
I1
1 2 3 4 5
27.38 27.46 27.38 27.26 27.38
...
6
Maximum variation., . . . . . . . . . . . . . 0 . 2 0 A V E R A G E . , . . . . . . . . . . . . . . . 27.37 I11
1 2 3 4 5 6
.
..
Vol. 14, No. 5
however, given consideration because repeated tests have shown that such discordant results continually occur and we typical of the methods. In fact, the rather close agreement of the majority of the results is to be regarded as a very insidious danger in forming a correct opinion of their value. Thus the series given, which includes three perfect checks, would indicate that with perhaps a little care in manipulation good results might be had. Only a thorough investigation of the procedures themselves and the careful repetition of many determinations have shown conclusively that consistent results cannot be obtained. Of the five methods given, 1,3, and 5 evidently agree much better with each other than with the other two or than the other two do with each other. (The agreement, of the results on Solution I by Methods 2 and 5 as well as those on Solution I1 by Methods 2 and 4 are fortuitous, as will be shown later.) This was rather unexpected, since Method 1 differs from Method 2 only in the strength of the solutions employed. The explanation, however, actually does lie in the fact that in the original technic of McCandless (Method 1) a strong solution of acid was used, i. e., 0.5 N; hence the “excess” was so large that the reaction was carried far enough to reduce the discrepancies between this method and 3 t o within the limits of experimental error. I n Method 2, on the other hand, the use of 0.1 N acid causes a smaller excess to be added beyond the phenolphthalein end-point, with the result that the figures approximate those obtained by 4 rather than by the other procedures. The uncertain point of the whole matter is the meaning of the term “excess of acid.” McCandless, of course, meant an excess over the amount of alkali not neutralized by the citric acid. He apparently assumed, and very naturally since this was the end-point ultimately sought, that this point would be shown by phenolphthalein. Unfortunately this is not the case, for on approaching it from the alkaline side a pseudo-end-point is first encount,ered a t which the color of the indicator is discharged although some of the alkali is still unneutralized. Only when the addition of acid is continued until the reaction of the solution is below a pH of about 5.5 is all of this excess alkali neutralized. The “excess acid” can then be titrated back with alkali and a true end-point with phenolphthalein reached. The proper reaction to be passed in the addition of acid can be determined by the use of methyl red instead of phenolphthalein as the indicator. Since methyl red is pink in acid solutions and straw-colored in alkaline ones it does not interfere with the use of phenolphthalein in the same solution.
I n order to make the relationships more clear a citrate solution was prepared and analyzed by Methods 2, 3, and 5; the courses of the tit
